Radiation curing of polymers is well known in the art. Though difficulties have been encountered in radiation curing or vulcanizing of polymers, improved results have been obtained by the use of additives. For example, in the case of polyethylene, it has been demonstrated that the addition of unsaturated polyfunctional monomers will produce comparable cross-link densities at reduced dose levels, in contrast to the radiation of ordinary polyethylene.
It is well known that polymers such as polyisobutylene are effectively destroyed when exposed to ionizing radiation. See for example, "Calorimetric Study of Radiation Crosslinking of Polyisobutylene In The Presence of Monomer Additives," Dokl. Akad, Nauk SSSR (Phys. Chem.) 193 (4), 855-857 (1970). The addition of small amounts of polyfunctional monomers such as p-divinylbenzene causes effective crosslinking of polyisobutylene as does the addition of acrylonitrile to polyisobutylene blends. Allyl acrylate has also been found to cause crosslinking of polyisobutylene under ionizing radiation conditions, see for example Polymer Letters, 2, pages 819-821 (1964). Although crosslinking does occur, degradation of the polyisobutylene results even in the presence of the allyl acrylates or allyl methacrylates at higher dose levels, e.g., above about 0.8 megarads.
Butyl rubber being substantially polyisobutylene also, not surprisingly, has been found to degrade in the presence of ionizing radiation.
The use of dithiols in the conventional curing of polymers is well known in the art. For example, the heat curing of a cellulose methacrylate derivative with simple dithiols is disclosed in British patent 588,018. Sulfur vulcanization of styrene butadiene is accelerated by the addition of bis-mercaptophenyl diphenyl oxide (see, for example, U.S. Pat. No. 3,326,822). Chlorinated butyl rubber has been vulcanized with dithiols. See, for example Hodges, Rubber Plastics Weekly 141 pages 666-668 (1964) wherein glycol dimercapto acetate is recommended because of its low odor.
The use of zinc oxide and stearic acid in the thermal vulcanization of polymers, and elastomers in particular, is well established in the prior art. See, for example, "Vulcanization of Elastomers", edited by G. Alliger and J. J. Sjothun, esp. pages 146-150 (1964, Reinhold Publ. Corp.). However, their unique capability for further improving curing using ionizing radiation when in combination with a hydrocarbon polythiol has not been previously described.
Elastomers have been prepared from long-chain dithiols with long-chain diolefins using as the crosslinking agent triolefin or long-chain trithiols; see, for example, Klotz et al., I and EC Product Research and Development 7, pages 165-169 September (1968).
In the present invention, it was surprisingly found that butyl type polymers containing allylic chlorine or bromine are radiation crosslinked rather than degraded in the presence of polythiols. In such cures, polythiols are unexpectedly more effective than dithiols. In contrast to the clean dithiol cure via addition of polydiene elastomers, the curing method of the present allylically halogenated elastomers, in the presence of polythiols, involves hydrogen halide elimination to a major extent.
It has also been surprisingly found that curing by ionizing radiation of olefinically unsaturated polymers is further enhanced by the combined presence of the components described herein; a hydrocarbon polythiol, a bivalent metal oxide and an aliphatic acid.